Automated blood pressure monitoring has become an indispensible tool in many aspects of medicine, perhaps the most important of which are those associated with the critical care patient. Increasingly, anesthesiologists utilize such monitors throughout surgical procedures, and the automated monitors are virtually universally used in critical care units, neonatal and adult intensive care units, and emergency treatment centers.
The most prevalent class of noninvasive blood pressure monitors utilize the so-called oscillometric method, and feature an inflatable cuff which is fitted over a limb of the patient, for example at the brachial artery. Through a complex system of inflation and/or deflation steps, the monitor senses arterial pressure changes and generates such information as mean arterial pressure, systolic pressure, diastolic pressure, and heart rate. Typically, the cuff size is adapted roughly to the anticipated arm size of the patients, and even though each typical cuff size permits some significant range of adjustability, the respective cuff sizes may be, on a volume basis, widely variant one from the other. Although given types of monitors operate with similar rationale regardless of the cuff size employed, it has been uncommon for any given unit to operate with more than one type or size of cuff and corresponding selective inflation volumes and pressures. Generally, this has been for the protection of neonates, who would be seriously injured if a cuff were to be inflated to the pressure levels required in the operation of sensing adult pressure. Hence, for the most part, commercial automated monitors have been designed to operate either for the neonate, or for adults, but not for both. Rarely, a given unit designed for both has been fitted with an operator actuated switch to select between neonatal and adult modes of operation; such units, however, have been regarded with considerable reluctance, since operator inadvertence could easily result in the application of the adult pressure parameters to the neonate.
It is a primary object of the present invention to provide methods and apparatus whereby automated blood pressure monitors may employ a variety of size ranges of cuffs, and which, automatically and without substantial dependence on operator diligence, will identify the type of blood pressure cuff actually to be utilized in the monitoring, and further will automatically adapt the system operating parameters to such type of cuff.
One prior art approach to such automated monitoring has been to measure the time required to inflate the cuff to a given pressure, or the time to inflate the cuff between two given pressures. In theory, the time required for such pumping is proportional to the volume of the cuff, and hence is indicative of the cuff size itself. In fact, however, artifacts resulting from line voltage changes, and variability of pump characteristics, often dominate the process, resulting in substantial rates of error in cuff identification. Another prior art approach has been to inflate the cuff to a given pressure, and then to measure the time required for a predetermined amount of deflation. Such an approach avoids artifacts caused by line voltage changes and pump variability, but encounters other difficulties. Specifically, any kinking in the hose or the cuff itself would result in substantial mischaracterization of cuff size. Indeed, such kinking is almost common in neonates, wherein the arm about which a cuff is applied is so small. Such kinking could easily prolong the deflation cycle, and cause an erroneous conclusion that the cuff volume is much larger than it is in fact.
It is a further object of the principles of the present invention to provide automated methods and apparatus for cuff identification which are substantially independent of line voltage changes and variability of pump characteristics, and which for neonates avoid errors caused by cuff blockages and kinking.